Among the fields in which superconductive wire materials are practically used, since the resolution of a high resolution nuclear magnetic resonance (NMR) analyzer is enhanced when a superconductive magnet used therefor is improved to generate a higher magnetic field, in recent years, a superconductive magnet generating a higher magnetic field tends to be further pursued.
As a superconductive wire material for forming superconductive magnets used for high magnetic field generation, an Nb3Sn wire material has been practically used, and for producing this Nb3Sn superconductive wire material, a bronze process has been primarily used. In this bronze process, after a plurality of Nb-based cores is buried in a Cu—Sn-based alloy (bronze) matrix, wire drawing is performed, so that the Nb-based cores are formed into filaments. Subsequently, the filaments are bundled to form a wire material group and are then buried in copper (stabilizing copper) for stabilization, followed by wire drawing. Next, the above wire material group is heat-treated (diffusion heat treatment) at 600 to 800° C., so that an Nb3Sn compound phase is formed at the interface between the Nb-based filaments and the matrix. However, since this process limits the Sn concentration solid-solved in bronze (15.8 mass percent or less), the thickness of the Nb3Sn phase to be formed is small, and the crystallinity is degraded; hence, there has been a problem in that high magnetic field properties are not superior.
As a method for producing an Nb3Sn superconductive wire material, besides the above bronze process, a tube process, and an internal diffusion process have also been known. In the tube process among those mentioned above, after an Sn core is disposed in an Nb tube, this tube is inserted in a Cu pipe and is then subjected to diameter reduction to form a wire, followed by heat treatment, so that Nb3Sn is formed by diffusion reaction between Nb and Sn (for example, see Patent Document 1). In addition, in the internal diffusion process, after Cu is prepared as a mother material, an Sn core surrounded with Cu is buried in the central part of this mother material, and a plurality of Nb wires surrounded with Cu is disposed around the Sn core, so that a composite is formed. Subsequently, this composite is subjected to diameter reduction to form a wire, followed by heat treatment thereof; hence, Nb3Sn is formed by reaction between Sn and Nb which are diffused in Cu (for example, see Patent Document 2). Unlike the bronze process, since these processes described above do not limit the Sn concentration which is solid-solved, the Sn concentration can be increased as desired, and hence the superconductive properties can be improved.
As a method for producing an Nb3Sn superconductive wire material, a powder process has also been known. For example, a process has been disclosed in Patent Document 3 in which after melt diffusion reaction is performed at a high temperature between Sn and at least one metal (alloy element) selected from the group consisting of Ti, Zr, Hf, V, and Ta to form an alloy or intermetallic compound thereof (hereinafter referred to as an “Sn compound” in some cases), the Sn compound thus obtained is pulverized into a powdered Sn compound, which is one of powdered raw materials, this powdered raw material is loaded as a core material (powder core portion which will be described later) in an Nb sheath or an Nb-based alloy sheath, and this sheath is subjected to diameter reduction to form a wire, followed by heat treatment (diffusion heat treatment) of the wire. Since a high-quality Nb3Sn phase having a thickness greater than that produced by the bronze process can be obtained by the process described above, it has been known that a superconductive wire material having superior high magnetic field properties can be obtained. In addition, by the process described above, it has also been shown that the Sn content in the powdered raw material can be increased to 20 to 75 atomic percent.
FIG. 1 is a cross-sectional view schematically showing the state in which an Nb3Sn superconductive wire material is produced by a powder process, and reference numerals 1 and 2 in the FIGURE indicate a sheath (pipe member) made of Nb or an Nb-based alloy and a powder core portion in which a powdered raw material is loaded, respectively. When the powder process is carried out, after a powdered raw material containing at least Sn is loaded in the powder core portion 2 of the sheath 1, the sheath 1 is then extruded for diameter reduction, such as wire drawing, to form a wire, and the wire is then wound around a magnet or the like, followed by heat treatment, so that an Nb3Sn superconductive layer is formed at the interface between the sheath and the powdered raw material.
As a heat treatment temperature for forming a superconductive layer, it has been believed that a high temperature of approximately 900 to 1,000° C. is preferable; however, it has also been known that when Cu is added to a powdered raw material, the heat treatment temperature can be decreased to approximately 650 to 750° C. From the point described above, in the powder process, after a small amount of powdered Cu is added to a powdered raw material, heat treatment is performed to produce an intermetallic compound, and in the tube process, a thin Cu layer is disposed inside the sheath. In FIG. 1, a single core is schematically shown by way of example; however, in practice, a multiple core material, which is formed of a Cu matrix and a plurality of single cores disposed therein, is generally used.
The superconductive wire material as described above is tightly wound to form a solenoid shape in many cases so as to be used as a high-magnetic-field superconductive magnet, and in order to prevent electrical short-circuiting in this type of tightly wound magnet, wiring is generally performed after an insulating material formed of glass fibers is disposed around the periphery of the wire material. In addition, beside a wire material having a circular cross-sectional shape, a wire material having a rectangular cross-sectional shape may also be used. In addition, since an Nb3Sn phase is very fragile, after the winding around a magnet or the like is performed, it is designed to perform heat treatment to produce an Nb3Sn phase (wind and react method (W&R method)).
As described above, it has been believed that the heat treatment temperature (diffusion heat treatment temperature) for forming a superconductive layer is preferably a high temperature of approximately 900 to 1,000° C. However, by the heat treatment at a high temperature as described above, glass fibers as an insulating material is embrittled, and after the heat treatment, sufficient insulating properties cannot be ensured. On the other hand, when the heat treatment temperature is decreased to approximately 750° C., diffusion of Sn from an Sn compound and reaction between Sn and Nb become insufficient, and as a result, superconductive properties (such as critical current density Jc) are disadvantageously degraded.
In addition, as described above, it has also been known that by addition of Cu to a powdered raw material, the heat treatment temperature can be decreased to 750° C. or less. However, when the configuration as described above is used, besides Sn and at least one powdered metal selected from the group consisting of Ti, Zr, Hf, V, and Ta, individual powders of Cu are each further weighed in an appropriate amount and are mixed together, and heat treatment is then performed, followed by a pulverization step. Hence, when the powder process is performed using a powdered raw material obtained by the steps described above, a very hard Cu—Sn compound is simultaneously produced in the heat treatment. The presence of this produced Cu—Sn compound causes abnormal deformation of the sheath during a diameter reduction step, and wire breakage may occur thereby in the worst case.
Furthermore, when a powdered raw material is loaded in a sheath material, a uniaxial press is generally used; however, instead of the treatment as described above, powder compaction by an isotropic pressure, such as a cold isostatic press method (CIP method), is preferably performed since the filling rate of the powdered raw material can be increased, and uniform processing can also be performed. However, when the CIP method is applied to the above powdered Sn compound, since the compound itself has inferior ductility, nonuniform deformation may occur conversely in a subsequent wire-drawing step, and as a result, a problem may arise in that production of a superconductive wire material itself becomes difficult.
Patent Document 1: Claims etc. of Japanese Unexamined Patent Application Publication No. 52-16997
Patent Document 2: Claims etc. of Japanese Unexamined Patent Application Publication No. 49-114389
Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-250749